A PROCESS FOR THE PREPARATION OF A SULPHONATED POLYMER MEMBRANE

Abstract

Abstract A method has been developed to synthesise an ionomer of sulphonated polystyrene ethylene butylene polystyrene (SPSEBS) using chlorosulponic acid as the sulphonating agent and to process the ionomer into a membrane. Proton conducting electrolyte membranes based on sulphonated Polystyrene (ethylene butylene) polystyrene triblock polymers were obtained by sulphonating the styrene blocks of low cost material, polystyrene (ethylene butylene) polystyrene. Membranes were prepared by solvent casting technique. Proton conductivity of such membranes was found to be excellent in the order of 10 S/cm in the fully hydrated condition at room temperature as measured by impedance spectroscopy. The sulphonated polymer chains contain the properties of both hydrocarbon block and ionomer block in their structure.

Full Text

COMPLETE SPECIFICATION
A process for the preparation of a sulphonated polymer membrane
Field of the Invention
The present invention relates to the synthesis and development of proton conducting electrolyte membranes that provides a functionalized polymer with ion (proton) exchange capacity
Background of Invention
Fuel cells are the subject of much attention during the last few decades mainly due to the environmental considerations, since polymer electrolyte membrane fuel cell (PEMFC) as power sources for automation generates electricity from a fuel source, where they convert the chemical energy directly in to electrical energy without any environmentally hazardous emissions such as NOx and SOx (typical combustion by products). Theoretically, the achievable energy of fuel cell is always greater than that of conventional engine, since the normal power generation process involves three steps: production of heat by burning the fuel; conversion of heat in to mechanical energy and conversion of mechanical energy in to electric energy.
The ion exchange membrane plays vital role in a solid polymer electrolyte fuel cell. It act as an electrolyte to provide ionic communication between the two electrodes that is; it facilitate the diffusion of ions (protons) from anode to cathode. The ionomer also serve as a separator for the two reactant gases that is; it prevent mixing of the fuel and oxidant gases.
In other words, the ionomer membranes, while serving as a good proton transfer membrane, also have low permeability for the gases, because the diffusion of gases across the membrane may lead to explosions due to high pressure and elevated temperatures of the fuel cell. In addition, to be an excellent conductor of protons, the membrane is also expected to be an electronic insulator, in order to avoid any short circuit and to extract the maximum power.

In a fuel cell, which consists of two electrodes, anode and cathode separated by an ion conducting membrane, the fuel (e.g. hydrogen gas) is ionized on anode to produce electrons and protons. The electrons travel through an external electrical circuit to the cathode, while the protons simultaneously follow an ionic path across the membrane to the cathode, where it combines with oxidant (e.g oxygen) to form water, electricity and heat.
Object of the invention
The main objective of this invention is to produce a membrane which can play the role of an ionomer in a fuel cell, by providing high ionic transport through them as well as prevent the cross-over of the molecular forms of the fuel and the oxidant gases.
An object of the present invention aimed to have zero electronic conduction, for the optimum performance in the fuel cell.
The second objective of the invention is that the membrane also be mechanically and thermally stable to withstand high pressure and temperature during the operation of a fuel cell.
In spite of several progresses, the PEM fuel cells have not yet been fully commercialized especially in automobile applications due to the high overall cost. Number of proton conducting membranes has been developed, but none of them possesses the required combination of high ionic conductivity, mechanical strength, chemical stability, dehydration resistance, and low fuel permeability along with a reasonably low cost.
Due to their high proton conductivity and good chemical stability, DuPont Nafion®, a perfluoro sulphonic acid polymer ionomer has been presently used as the typical PEM in a fuel cell.
The major disadvantages of these membranes are their high cost, difficulty in manufacturing and recycling processes and the permeation of methanol and ethanol from anode to cathode.
Hence, the third objective of the present invention is to produce an ion conducting membrane from low cost materials with a hydrocarbon base.

Another objective of the invention is to easily sulphonate and synthesize the ion conducting polymer from the available polymer materials using common sulphonating agents. Preferred sulphonating agents for these methods include sulphuric acid, methane sulphonic acid, benzene sulphonic acid and chloro sulphonic acid.
It is also an objective of the Invention to provide a process for preparing the ion-conducting polymer by a simple technique, the method which can be performed easily.
Therefore, it is highly desirable to develop an inexpensive ion-conducting membrane with high proton conductivity, good mechanical and chemical stability. The membrane is also desired to have good film -forming capacity with minimum thickness.
Brief description of prior art
us Pat No 5422411 discloses the use of poly (trifluorostyrene) copolymers as polymer electrolyte membrane for fuel cell. These membranes have poor mechanical and film forming properties, in addition to their high cost, which may arise due to their fluorine content. Hence, it is another objective of this invention to produce an ion-conducting membrane, which can be easily processed to form films.
WO 97/24777 discloses the solid polymer membranes based on sulphonated poly 2, 6 - dimethyl - 1, 4 - phenylene oxide blended with PVDF. These membranes are known to be vulnerable to dehydration form peroxide radicals. Hence, a further objective of this invention is to produce a stable ionomer membrane which is not vulnerable to dehydration.
US Pat No 3577357 discloses the preparation of a sulphonated styrene -(ethylene - propylene) copolymer by selectively hydrogenating styrene isoprene styrene triblock copolymer and then sulphonating using SO3/TEP reagent at 60°C for 1.5 hours. But these ionomers could not be cast into films and no membranes were produced.
Gray et al (Macromolecules 21, 392-397, (1988)) discloses a styrene butadiene styrene block copolymer based membrane and found that this morphology does not facilitate the ion-conductina orooerties that are necessary for fuel cell

operation. But the styrene block only functions as a mechanical support structure for the polymer.
Several block ionomers are being studied recently due to their unique morphologies arising due to the combination of both hydrocarbon block and ionomer block in their structure. They also club the advantage of being low in cost, when compared to the fluorine based polymers.
Zhang et al (European polymer journal, 36, 61-68, 2000) have reported the synthesis of diblock copolymer ionomer based on polystyrene block ethylene copolymer using acetyle sulfate as the sulphonating agent. Conductivity measurements were not carried out.
Basher et al (solid state ionics, 139, 189, 2001) and Mokrini et al (Polymer, 42, 9, and 2001) reported the preparations of hydrogenated poly styrene butadiene block copolymer blended with polypropylene for use as ionomer membranes. Though the membranes show good dimensional stability due to the polypropylene content, the conductivity properties are reduced since polypropylene does not participate in the proton conduction other than imparting mechanical strength.
US Pat Nos 5468574 (1995) and 5679482 (1997) discloses the preparation of sulphonated styrene (ethylene - block - butylenes) styrene polymer using SO3 in dichloro ethane medium. The proton conductivity of this membrane is claimed to be in the order of 10"^ S/cm in it's fully protonated state. The reported conductivity values are very less for the fuel cell to perform at its best.
Whereas, the present invention relates to the preparation of an improved proton conducting polymer membranes obtained from low cost materials and are useful for applications as solid PEM in a fuel cell. These membranes exhibit excellent proton conductivity better than the standard membranes, which are commercially available.
Summary of the Invention
The present invention is to provide an alternate for the currently used Nafion® proton conducting membrane. The main objective Is to produce a proton conducting polymer membrane having high proton conductivity along with appropriate methods for their preparation. The membrane is aimed to have other improved properties

including high resistance to dehydration, high mechanical strength, chemical stability, low gas permeability and stability at elevated temperatures and pressures.
The present invention is very helpful to reduce the overall cost of producing polymer electrolyte membranes, so as to give way for a successful commercialization of fuel cell vehicles. Hence, it is an objective to synthesise a proton conducting membrane from commercially available low cost starting polymers.
It is a further objective to prepare defect free conducting films of low thickness (approximately 100 microns) with high strength. Preferred thickness of the sulphonated membranes for the application to fuel cells is 100 to 500 microns.
The membrane is also have good dimensional stability and a membrane which is not brittle in both dry and wet forms. The membrane to be capable of retaining considerable strength even after absorbing a significant quantity of water.
The proton conducting membrane is very stable to temperatures of at least about 100°C. The prepared ion conducting polymer membrane of the present invention is aimed to be swellable but highly insoluble in boiling water over long time periods.
The present invention is to produce proton conducting membrane having high ion exchange capacity (lEC) of greater than about 0.1 S/cm, more preferably in the order of 10'^ S/cm or less than about 10 Dcm resistivity.
The prepared ion conducting polymer materials are easily cast in to films with out defects and shrinkages. The prepared sulphonated polymer is also expected to dissolve in a common solvent or a mixture of common solvents. Preferred solvents for these methods Include tetra hydro furan (THF) dichloromethane (DCM), chloroform, 2-propanol, 2-butanol, toluene, dichloro ethane or a mixture of two or more solvents mentioned here.
The present invention is to choose the starting polymer material from several of the commercially and cheaply available block polymers based on hydrocarbons, since, their sulphonated products can have unique morphologies arising due to the combination of both hydrocarbon block and ionomer block in their structure. The flexibility of the membrane is provided by the hydrocarbon content of the material, while the ionomer character is facilitated by its styrene content of the polymer.

Preferred polymers include polystyrene-block-polybutadiene (30 wt% styrene), polystyrene - block - poly(ethylene - ran - butylene) - block -polystyrene, (styrene content 29%, Mw 89000) and polystyrene-block-polyisoprene-block-polystyrene (14 wt% styrene). The styrene (ethylene-butylene)-styrene tribiock polymer is preferred to have a weight average molecular weight of more than about 50000 and styrene units may constitute from about 25wt% to 35wt% in the copolymer, and preferably the polymer is sulphonated to more than about 30% of the styrene units.
The highly conducting sulphonated polymeric membranes are formed by
(a) Dissolving a block polymer (about 5g) in a suitable solvent (50ml) to form a polymer solution,
(b) Maintaining the temperature of the reactor vessel at -5 to 0°C,
(c) Stirring for five to ten minutes at -5 to 0°C,

(d) Adding tributyl phosphate (about 0.5 to 1 ml) to the above solution and continuously stirring for another 5 to 10 minutes,
(e) Then adding 0.5 to 2 ml of the sulphonating agent to the reaction vessel maintained at -5 to 0°C with continuous stirring,

(f) Allowing the reaction to proceed for half hour to 2V2 hours, where the colour change occurs from transparent to yellow then to light brown and further to dark brown colour,
(g) Adding more than about 50 ml of a lower aliphatic alcohol, after the required reaction time,
(h) Continued stirring for another 4 to 5 hours,
(i) Evaporating the solvents at 65 - 70°C overnight,
(j) Number of washings by water followed by washing for one hour in boiling water to completely remove any residual acid from the product,
(k) Again several washing until a neutral pH,
(I) Drying the product at 75°C for one day,
(m) Dissolving the sulphonated polymer in a suitable solvent.

(n) After about 24 - 30 hours with a continuous agitation casting the polymer solution on a casting surface,
(o) Allowing the solvents to get evaporated slowly in a controlled manner for 24 hours, so as to form the polymer membrane with uniform thickness, without any wrinkles and defects.
The prepared membranes are widely useful in a variety of electrochemical devices, particularly in a electrolytes in hydrogen oxygen fuel cell. Polymer Electrolyte Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC) and other electrochemical devices such as water eiectrolyzer and sensors.
Synthesis of sulphonation of polystyrene ethylene butylene polystyrene

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Experimental Details
Following are the procedures explained in the preparation and testing of the sulphonated polystyrene ethylene butylene polystyrene membrane samples.
Sulphonation
A number of experiments were performed to carry out the sulphonation reaction of block polymers. Various sulphonating agents like sulphuric acid and chlorosulphonic acid were used. Degree of Sulphonation is controlled by the choice and amount of sulphonating agent used, by the choice and amount of solvent (polymer concentration) and by the reaction time.

The required amount of polymer preferably polystyrene ethylene butylene polystyrene (PSEBS) is first dissolved in a required amount of a suitable solvent preferably chloroform and then allowed for stirring at a temperature within the range of from -10 to +15°C, for 10 - 30 minutes. Required quantity of tributyl phosphate is added to the reactor vessel. Sulphonation is carried out by introducing suitable sulphonating agent, preferably chloro sulphonic acid. The amount of sulphonating agent preferably 0.1 to 0.3 ml should be sufficient to introduce a number of sulphonate groups in to the polymer chain. The temperature of the reaction mixture is maintained at -5 to +10°C with continuous stirring.
After the desired degree of sulphonation has been reached, the sulphonated polymer was further allowed to continuous stirring (1-3 hours) after adding an excess amount (5-15ml/g) of any lower aliphatic alcohol into it. The sulphonated polymer product was separated out by allowing the solvent to get evaporated. The solvent can be removed simply by evaporation. The obtained solid product was then subjected to the conventional methods such as washing and drying. The complete removal of residual acid is very critical; finally the ionomer (product) is dried in an oven at 60°C for one day.
Membrane Preparation
Required amount of sulphonated polymer (Ig in 10ml) is soaked in solvent, and then the dissolved polymer solution is cast in to the clean glass plate, and allowed for slow evaporation of the solvent. After the complete evaporation of the solvent, the dry membranes are recovered from the glass plate by peeling.
Membrane Purification
The dry membranes were purified by boiling for half an hour for each step in 3% H2O2, in 0.3M H2SO4 and in water before use for further studies.
Sulphonation confirmation by FTIR
l=TIR spectra were recorded in absorbance mode on a Perkin - Elmer FTIR spectrometer. The spectra were performed for sulphonated and un-sulphonated membranes.

Ion Exchange Capacity
Ion exchange capacity (lEC) of the acidified membranes determined by titration method as mentioned below.
1. Cut out a piece of sulphonated film (weight about O.lg).
2. Dry the film at 75°C for one day and weighed for dry weight.
3. A weighed film was placed in a beaker filled with a 2M KCI solution and the salt solution after boiling for five minutes is titrated against O.OOIN Na2C03 solution to a phenolphthalein end point and lEC (meq/g) was calculated from the volume of the NaaCOs solution consumed to obtain a neutral pH.
Proton Conductivity
The proton conductivity of the membrane is measured by AC impedance spectroscopy. The conductivity is measured by sandwiching the membrane sample between two stainless steel electrodes. The electrodes and membrane samples are fixed by blots and nuts. The conductivity is calculated from,
a = L / RA
Where, o - proton conductivity, L - the thickness of the membrane, A - area of the membrane and R - the bulk resistance value measured.
Water absorption
To determine the water absorption, first the membranes were dried at 75°C for 24 hours and then weighed for its dry weight; samples were immersed in water at room temperature. The membranes were blotted dry to remove the excess solvents that are on the surface and weighed at different times. The water uptake and linear expansion of the membranes were calculated using following formula.
Ww-Wd
S = X 100 (%)
Wd
Where, Ww and Wd are the wet weight and dry weight of the membrane respectively.

Thermal stability TGA and DSC
The TGA is carried out using a thermo balance SDT Q6G0 US analyzer at a heating rate of 20oC/min under nitrogen atmosphere.
Leaching Test
Leaching experiment is performed in order to confirm the proton conductivity of the membranes even after storing them in water for many days. The water was freshly changed everyday. Conductivity of the membranes was tested at regular intervals of time.
Results and discussion
IR
Figure 3 shows the IR spectra of un-sulphonated and sulphonated PSEBS samples. Appearance of bands at 1007 cm~S 1031 cm~^ and 1126 cm~^ in the sulphonated sample indicated the sulphonation of the phenyl ring. The disappearance of peaks at the frequency of 699 cm"^ and 760 cm~^ on sulphonation indicated the substitution in the aromatic ring when compared to the un-sulphonated sample.
Ion Exchange Capacity
The lEC of the sulfonated PSEBS membranes was found to be in the range of 0.5 to 3.0 meq/g. lEC provides an indication of the concentration of the number density of sulphonic acid groups present in the polymer matrix, which are responsible for the conduction of protons and thus is an indirect and reliable approximation of proton conductivity.
Proton conductivity
The proton conductivity values are tabulated in Table 1. It was found that the proton conductivities of SPSEBS membrane are excellent in the range of 10"^ S/cm.
Water absorption
Water absorption of the membranes is usually defined in weight percentage with respect to the weight of the dry membrane. The absorption of water depends on the lEC. Higher the lEC, greater is the absorption of the solvent. Initially the

membrane rapidly absorbs the water and then gradually increases with time. In fact, in case of immersion in water, the SPSEBS polymer membrane easily expanded by absorption of water.
The water absorption of the membranes was found to be in the range between 60 and 180%, which increases with the lEC of the prepared samples. lEC of the ionomer was modified by varying the reaction time, and the sulphonating agent concentration.
Thermal stability
Figure 4 shows the DSC curves of the samples studied. The inflection point of the slope change of the heat capacity plot was taken as the Tg and it was around 71 °C for the un-sulphonated polymer and about 98 °C for the sulphonated one. Hence, sulphonation increases the Tg of the polymer due to the larger sulphonic acid group present in the sulphonated polymer.
The TGA curves of the polymer samples are shown in figure 5. The thermal decomposition temperature of the membrane is a function of sulphonation. The un-sulphonated sample display thermal stability up to 350 °C, but the sulphonated membrane loses its stability beyond 250 °C. A small transition around 100 °C in the sulphonated and sample is attributed to the presence of moisture.
The transition around 250 °C may be due to the thermal degradation of sulphonic acid groups. The transition around 400 °C is attributed to main chain scission. Thus, sulphonation decreases the thermal stability of the polymer due to the degradation of the sulphonic acid groups at lower temperatures. However, the sulphonated hydrocarbon polymer membrane has adequate thermal properties for application in fuel cells, since their thermal decomposition is detected only above 230 °C.
Leaching Test
It was observed that, there was not much change in the conductance value of these ionomer membranes, when stored in water for a period of 1 month at room temperature. The membrane was tested for conductance and washed, dried and soaked in fresh de-ionized water every day. The conductance value was found to be stable in the same order. The sulphonated polymer membranes are found to be active enough in conduction and the properties of them were not lost even after a

period of 1-month storage in water. This confirms that the acidity is not due to any surface acids or loosely attached acid groups and also that there is no loss of active sulphonic acid groups, attached to the polymers that are responsible for the protonic conductivity.
EXAMPLES:
Example 1
Sulphonation of PSEBS using cholorosulphonic acid
A three necked 250 ml round bottom flask equipped with an addition funnel, condenser and overhead stirrer were charged with 5 g of the triblock copolymer, polystyrene ethylene butylene polystyrene with a styrene content 29wt% Mw = 89,000 obtained from Aldrich and 50 ml of chloroform (from Merck). This mixture was stirred for approximately 15 minutes until a solution was formed.
After the formation of a clear solution they were cooled in an ice bath to about a temperature of 0°C. Ice bath and continuous stirring was maintained throughout the duration of the reaction. After about 15 minutes, when the solution also reached the temperature of the ice bath, 1ml of TBP (to moderate the reaction) was added in to the reactor. The content was further allowed for continuous stirring for about five minutes.
To the rapidly stirring, cooled and dilute polymer solution, 1 ml of the sulphonating agent, chlorosulphonic acid was added drop wise through the addition funnel over a period of 5-10 minutes. After the complete addition of the sulphonating agent, the mixture in the reactor vessel was permitted to stir continuously at ice bath temperature for 2 hours of reaction time. After the completion of the required reaction time, the sulphonation reaction was terminated by adding an approximate amount of 25 ml of methanol.
The reaction mixtures were further allowed for continuous stirring for about 5 hours. The reactor vessel contents were then transferred into a tray, and the polymer was concentrated by allowing the solvents to completely get evaporated until the contents were converted into a solid sheet form.

The product obtained was then washed with water several times. The washed products were then dried at 75°C for one day. About 5 g of the sulphonated product were dissolved in about 50 ml of THF. After one day, the contents resulted in a highly viscous solution.
The membrane cast from this solution was subjected to various tests. Following are the several of different properties of the membrane. The membranes were also purified by washing in 3% H2O2 and dilute sulphuric acid.
Other sulphonation reactions were also carried out similarly as described above, with one or more changes, appropriately as mentioned.
Example 2
Using the sulphonation method as described in Example 1, a sulphonated polymer was prepared using 2 ml chlorosulphonic acid.
Example 3
Sulphonated polymer was prepared by using the procedure with 2 V2 hour reaction time.
Example 4
The method of Example 1 was employed in this experiment; a sulphonated polymer was prepared with 1 hour reaction time.

Figure (2) is a schematic representation of the structure of the sulphonated polymer in the hydrated condition.
Figure (3) is a IR curve for sulphonated and un-sulphonated PSEBS
Figure (4) is a DSC curve of the polymers
Figure (5) is a Thermo gram of the polymers
Detailed description of the Drawing
Referring to Figure (1), it shows schematically the working of a fuel cell comprising an ion exchange membrane (6), positioned between anode (1), and cathode (2). i^embrane (6) can be a cation permeable membrane, having protons (5) as the mobile ion and the fuel gas can be hydrogen (3). In this reaction, gaseous molecular hydrogen (3), is ionized to hydrogen ion (5), migrating from anode (1), to cathode (2), through membrane (6) and electrons (4), migrating through external circuit. In this case the overall cell reaction is the oxidation of hydrogen (3) with oxygen (7) to water (8).
Referring to Figure (2), shows a schematic representation of the structure of the sulphonated polymer chain (1) along with sulphonic acid group (2) attached to the chain in its hydrated (water 4) condition along with proton (3).
Referring to Figure (3), shows the results of FTIR analysis performed for sulphonated and un-sulphonated polystyrene ethylene butylene polystyrene ((2) and (1) respectively) to identify the structure of ionomer according to embodiment of the present invention.
Referring to Figure (4) and (5), represents the DSC curves and Thermo grams of the sulohonated (2) and unsulphonated (1) polymers to confirm the thermal stability of the prepared membranes.

We claim:
1. A process for the preparation of a sulphonated polymer membrane comprising: sulphonating the styrene units of the polymer using chlorosulphonic acid wherein Dissolving a block polymer (about 5g) in a suitable solvent (50ml) to form a polymer solution.
2. A process according to claim (1), wherein the said polymer is polystyrene - block - (ethylene - ran - butylene) - block - polystyrene.
3. A process according to claim (2), wherein the proton conductivity of the sulphonated polymer membrane is from about 0.01 S/cm to about 0.1 S/cm, wherein the ion conducting polymer has an ion conductivity >0.1 S/cm.
4. A process according to claim (1), wherein the lEC is from about 0.5 meq/g to about 3.0 meq/g, wherein the ion conducting polymer has an lEC > 1.0 meq/g.
5. A process according to claim (1), wherein the sulphonated polymer is in the film form,
6. A process according to claim (1), wherein the polymer, polystyrene - block -(ethylene - ran - butylene) - block - polystyrene is sulphonated, phosphonated or carboxylate.

7. A process for the preparation of a polymer membrane from the sulphonated
polymer obtained from claim (1), comprising of dissolving the polymer in an
organic solvent to form a polymer solution and further casting into membranes.
8. A process according to the claim (1), wherein the solvent used for preparation of
membrane is tetrahydro furan.
9. A proton conducting polymeric membrane made by any of the methods of claims
(l)-(8).
10. A fuel cell comprising the proton conducting polymeric membrane of any of the
claims (1) - (9).